Last Sunday night I was at a reception that required a backdrop for larger groups. I didn't have any radio remotes to fire the lights, so I thought about using the Ultra Zap 2400's built in optical slave. I thought I could use my SB800 and swivel it to point backward towards the Ultra Zap and use it to trigger the Ultra Zap. But the normal setting on my SB800 is TTL and it sends out a little pre-flash to determine the exposure. That little pre-flash triggered the Ultra Zap so that when the actual exposure was made, the Ultra Zap was recycling and did not fire.
Setting my SB800 to manual and dialing it down solved the problem. Manual doesn't give you a pre-flash, so only the actual exposure flash triggered my off-camera optical lights. Situation resolved.